Method of making a shell mold by lost wax process



3 Sheets-Sheet 1 I AIR DQY I I AIQ DQY AIR DRY DQEHEAT ,mmum'n .1 I'H'IIIIIIIII STUCCO COAT 2 DI COAT STUCCO COAT EUTECTIC COAT DRAINAGE T. OPERHALL ETAL METHOD OF MAKING A SHELL MOLD BY LOST WAX PROCESS Original Filed Jan. 13, 1958 DIZAINAGE DQNNAGE X DIPCOAT DQAINAGE DRAINAGE POST FIRED Dip COAT METAL POUIUN July 27, 1965 MAX cLusTEl2|-[m DIP COATI I AIR DIZY I I Am DIZY I name WAX REMOVAL comma Theodore Operhall C bar 165 W. Schwarfs Dlckson Van Schoik INVENTORf) CLEANING 0M, W Jmyefi, WM -7M ATTOPDELIS y 27, 1965 1-. OPERHALL ETAL 3,

METHOD OF MAKING A SHELL MOLD BY LOST WAX PhocEss Original Filed Jan. 13. 1958 5 Sheets-Sheet 2 DIP COAT FINE PARTICLES CONTINUOUS MATRIX OF BINDER FINEFZ PARTICLES Theodore OperhaH Charles W. Schwarfa D|ck5on Van Scholk INVENTORS y 1965 1'. OPERHALL ETAL 3,196,506

METHOD OF MAKING A SHELL MOLD BY LOST WAX PROCESS Original Filed Jan. 13, 1958 5 Sheets-Sheet 3 I z mmwm O m m 0. m 5 0 QM mwm s n wm mmm T INVENTORS AHo rna g5 United States Patent 3,196,506 METHOD 0? MAlKiN-G A SHELL MULD BY LGST WAX PRUCESS Theodore Uperhalll and tlharles W. Schwartz, Whitehall, Mich, and Dickson Van Schoil-r, Salem, Ohio, assignors, by mesne assignments, to Howe Sound tlompany, New York, NSC, a corporation of Delaware (iriginai application tan. 13, 1958, Ser. No. 708,628, now Patent No. 2,961,751, dated Nov. 29, 1960. Divided and this application Nov. 8, 1960, Ser. No. 68,075 3 Claims. (Cl. 22-196) This application is a division of our copending application, Serial No. 708,628, filed January 13,1958, and entitled Metal Casting Process and Elements and Compositions Used in Same, now Patent No. 2,961,751, issued November 29, 1960.

This invention relates to the fabrication of metal castings, and it relates more particularly to a metal casting process and materials for use in same embodying some of the elements of an investment casting process with some of the elements of a shell-molding process while eliminating many of the limitations of each of the processes to provide a new and improved procedure for the production of metal castings.

Both the investment casting process and the shell-molding process are well known to those skilled in the art such that detailed description thereof by way of introduction shall be unnecessary here. Sufiice it to say that the investment casting process finds objections in the field from the standpoint of thehigh cost of materials required in the preparation of the investment and mold for easting; in the large amount of highly skilled labor required for preparation of the wax patterns and the mold, and in the utilization thereof in the casting process, and in the high percentage of scrap which generally characterizes the investment process for the production of metal castings thereby to prevent reduction in cost of castings so produced.

While shell molding with an organic resinous binder eliminates or rather minimizes some of the more objectionable features of the investment casting process, such shellmolding process, as it is practiced today, has other characteristics which are objectionable from the standpoint of the desire .to produce acceptable castings at high yield with a minimum amout of labor, materials and cost.

Thus it is an object of this invention to provide a new and improved method for the production of metal castings, and it is a related object to provide new and improved materials for use in the practice of same.

More specifically, it is an object of this invention to provide a new and improved metal casting process which, while embodying some of the desirable features of the shell-molding and investment casting processes, eliminates many of the undesirable characteristics of each and, in addition, provides means whereby a higher yield of good castings can be achieved at less cost from the standpoint of the cost of materials, the amount of labor, time and space, as well as equipment required for manufacture.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration, but not of limitation, an embodiment of this invention is shown in the accompanying drawing in which:

FIG. 1 is a fiow diagram of the process embodying the features of this invention starting from the cluster of wax patterns to the completion of the mold for pouring the metal castings;

FIG. 2 is a perspective elevational view in partial section of a wax cluster employed in the practice of this invention;

FIG. 3 is a perspective view of the cluster shown in FIG. 2 with the coatings applied thereon in accordance with the practice of this invention;

3,1965% Patented July 27, 1965 FIG. 4 is a perspective view similar to those of FIGS. 2 and 3 of the shell mold after the baking operation to remove the wax and to mature the ceramic materials; and

FIG. 5 is a schematic sectional elevational view of the metal melting furnace and the mold clamped thereon.

The invention will hereinafter be described with reference to an illustrative practice of the invention, it being understood that variations may be made within the skills of the art.

Preparation of the cluster To the point where a cluster v10 is formed of the wax patterns 12 mounted between wax runners 14 in communication with a wax spout in the form of a crucible 16, the process is almost identical with the conventional investment casting processes as defined in the Collins Patent No. 2,380,945, Feagin et a1. Patents Nos. 2,439,207, 2,441,695 and 2,521,839, and in the Operhall Patent No. 2,806,271.

Dip coating composition 8000 cc. colloidal silica 30% grade) (specific gravity 1.198)

165 pounds zircon (99% through 325 mesh) (65-67% ZnO 3432% SiO 6150 cc. water.

110 grams sodium fluoride Application of first dip coat The cluster 10 of wax patterns 12, carried by a handle embedded in the crucible defining the pouring spout 16 of the cluster, is first inspected to remove dirt, wax flakes and other objects which may have adhered to the surfaces of the wax patterns and, if allowed to remain, would impair the preparation of a good mold, thus leading to an imperfect casting. The cleaned cluster is then immersed into the slip in a manner to coat all of the surfaces of the cluster with the exception of the lip of the crucible. To promote the removal of bubbles from the cluster design, it is desirable to rotate the cluster while immersed in the slip.

When fully coated, the cluster can be removed from the tank to drain. During drainage, the cluster can be inspected to detect air pockets which can be eliminated by directing a stream of air onto the deficient portions and thereafter allowing the slurry of the dip coat to flow onto the blown area. While the cluster is .being drained, it should be held in different planes designed to achieve uniform coating on all surfaces. In general, drainage should be completed within a few minute but, in any event, in less time than would allow the dip coat to gel or dry to the extent that a good stucco coat cannot be applied.

F irsz stucco coat Stucco combinationAlundum (100% through mesh with less than 3% through 100 mes-hbetter than 90% and mesh) Application of stucco coat After the uniformity of coating has been achieved in the first dip coat and dripping from the patterns has become minimized, the stucco is sprinkled onto the wet coating of the cluster substantially uniformly to cover the dip coat with a layer of the stucco while, at the same time, minimizing flow of the dip coat whereby non-uniformities might otherwise develop. In practice, the stucco particles will be rained down from above through a screening member constantly being fed by a vibratory feeder to remove foreign matter from the Aludum particles while the particles are sprinkled over an area to give more uniform and complete coverage. The stucco will adhere to the wet coating of the slurry and will become partially for a short time to set the binder.

embedded in the slurry to become integrated" with the coating formed on the cluster of wax patterns.

If the dip coat is adjusted to enable gelat-ion to take 7 place within a very short period'. of time, greater. than the time required for drainage and stuccoing,,thev stucco cluster need 110L136 set aside for'drying. "However, itis preferred to slow. the gelation 01 1116 dip coat so. thatsuffi-j cien-t leeway is available for the desired drainage and,

stucco application and to enable correction. fordeficiem cies which might exist with respect to. one or theother Thus, it is desirable to provide for an, air. dry in the amount of from -25 minutes and preferably from 10'- minutes.

minimums described without harm to the. structure.

' Second dipcorzt l The composition ofthe second dip coat, sl-ipissub stantially the same as theafirst. dn practice,-use isusualto provideaseconddip coaton the stuccoed, cluster'isf achieved in a manner similar to that of the previous first;

1y made. of the first dip coat slip., Application of the slip dipcoat. "*1; Second stucco coat.

Stucco combination-Alundurn 3'-60 mes'l1 than 3% through 70 mesh) Further dipcomjslips;

i The compositionof the third and additional dip coat Itwill be understood thatthe time of dry-; ing may be extended inde'finitely beyond the preferred 165 pounds zircon, (99% 5-10 or more dipsandstucco coats of the type described may be applied in series onto the wax cluster.

Eutectic com position 8000 cc. colloidal silica (60% grade) (specific gravity through. 325 mesh). (6 5-67% Z110 '3 +3'2% .SiO I v Q 6250 cc; water 1 15 grams sodium fluoride 18.5 poundsfeldspar:

; CaO,' '6.%"

Q,J A1 0 161 9 SiO 60-63% 7 a The. eutectic coating comp'osition is similar in formulation'and'. preparationto'the previously defined dip coat Theieutec-tic coating is applied in a manner Quitesimi 'With the exception of the feldepar which is' added together, with -the:zirconi flour to the mixture.

lar to the dip coats. The st-uccoclustcr 'is immersed in the: slip with relativemovement between the slip and cluster to achieve more uniformidistribution and to elimimate as much of theiair pockets as-possible, The coated --c1usteris :then rern oved fromthe slip andthe excessjeutectic'coat-ing composition is allowedto drip from the cluster while turning the cluster to secure a desired vuniform slips correspondsito the. composition of thefirst and seconddip coat slips. Use is usually made of'the first or I T seconddip coatslips which probably have become -con-' taminated by thepresence of'coarse particles of a-lundu-m or-stucoo removed from' the cluster during previous dips.

This'is because a slipwith coarse'particles of the type described would be unsuitable for use as a rfirst dip'coat I j because of the undesirable effect that the coarse. particles would have on the mold surface and on'the casting produced withinthe mold, In commercial practice, where the dip coat compositions will be consumed at a relatively rapid rate, the scparation a's between the first and subsequent dip. coats is not so important; but it is preferred to go up the scale rather than down if any change's are 7 made.

The third and additional dip coat isachieved essen tially the same manner as the first andseconddip coats by:

immersion of the, cluster with the first and second stoucco;

coats into. the slurry while turn-ingjthe cluster or-by re volvingthe drum in which the sl-urryis contained. After coating on'the surfaces. V V V *Aft'er; the eutecticflcoating has been suitably drained,

.it' is stuccoed as in the additional stucco coats described and then the cluster is allowed to air dry. Sufiicient air drying will take place within 16 hours atam'bient temperature and the d-ryingtime can be shortened considerably by using a dehumidified atmosphere. 7 It is possible under suitably controlled conditions completely todispense withthe airdrying step" and proceed almost immediately to the baking or. firing operation in't'he prepara tion of the mold, but it is preferred to make use of a drying and aging period sirfficient, at-least; to remove free moisture, when present. r V p Firing ihe cluster 1 The cluster having the predetermined number of dips and stucco coats and oneor more of the eutectic coats on the surface isjsubsequentlyfired ,byexposure. to a temperature 0]?1800". F. forfrom 3-30 minutes, depending.

greatly upon the mass" of -material which. is being subjected to theiiring and baking operation. The cluster is.

usually suspended iwith' the crucible facing' downwardly 111 the furnace so thatthe wax canv drainv from. the mold asit is'reduced at theseten'ipe'ratures'toa molten state.

coating, the cluster is drained while tu rning to provide a:

uniform coating of the slipover the entire surface with the exception of the lip 'of'the crucible.

7 Further stuccocoats u Stuccotabular alumina For the third andadditional stucco, coats, useis prefer ably made of tabular aluminain the form of coarse particles ground .to pass through a 14-rnesh sieve and .to be retained on a 2 0-mcsh sieve with less than 10% through a SO-mesh sieve and less than 1% on a 6 mesh sieve.'

Application is made as before onto the wet layers of the previous dip coat after proper drainage and distri-bu-- tion. After each coat, the coated 'wax pattern or 'clusten' of patterns is allowed to dry pr set to the gelled stage prior to the next series of coatings; If gelation' occurs almost i-mmediately'upon application of. the stucco coat,

.The great majority of the wax from thepatterns and parts will beremoved .by drainage while in' the molten "stateand the residue'which remains in'the mold will be i rapidly burnedout atthe temperature conditions existing.

The fired productl can thereafterbe set' aside to cool. The hp willflbegrounddown to provide a flattened surface a'daptedfmore substantially toconform with the face plate of an invertible furnace. in which the metalis reduced to themolten'state so thatthefinishedmoldcan be clamped to the faceplate of the. furnace to enablethe metal to be poured directlyinto the shell mold by inversion. Inpractice, it is preferred .to' grind down the lip before the fir ing step.-

' WhiIeasheIl mold suitable.forpouring wili be secured at the conclusion of'thefiring step used to free the shell of the wax patterns, gates and other parts, and to ma 'ture the. ceramic :niaterials'making 'up the shell mold, fsome expansions and contractions 'do i take place which may resultin the formation of'srnall cracks and fissures. Thus itisfpreferred; in accordance with the practice of .aconcept of this invention, to provide for additional vitri-" the dryingstep mayibe dispensed with. Usually. from '75 fication in the outer portio'nqof the rnold'and to'increase' i like.

the strength of the mold while at the same time sealing the cracks and fissures by the application of another eutectic coating to the outer surface of the shell mold subsequent to the firing operation. This builds up an additional concentration of feldspar and the like lower melting ceramic which is barred by the previous layer from deep penetration of the mold and thus remains more concentrated on the surface where it can provide vitrification and where it can heat seal the pores or cracks in the mold.

The post-fired coat corresponds to the eutectic coating and application is the same as in the application of the eutectic coat prior to the firing operation. When a postfire coat is applied, the coated shell is set aside to dry and the ceramic is matured as an incidence to preheating of the mold to the temperature desired for pouring. For this purpose, the mold is usually preheated to a temperature within the range of 1600-2200" F. and preferably to a temperature within the range of 17001900 F., and is then clamped to the furnace for turning over to pour the metal into the heated mold. One of the novel characteristics of a mold prepared in accordance with the practice of this invention resides in the ability to preheat the mold for an extended period of time, as for days, at the temperature conditions described without noticeable effect from the standpoint of slumping or the type of vitrification which is characteristic of systems heretofore employed.

Pouring Prior to pouring the mold can be very carefully and completely inspected to eliminate imperfect molds which might otherwise produce metal castings incapable of use. The ability carefully to inspect the molds at this stage, prior to pouring, materially reduces scrap loss by comparison with the investment casting process wherein the mold cannot be inspected so that any imperfections in the mold will remain hidden until their presence is disclosed by the unacceptable castings that are produced.

Molds prepared in accordance with the practice of this invention are characterized by sufiicient strength to enable use as a shell without investment to be clamped as an investment mold to the furnace for pouring. The mold can be clamped to the furnace without additional support to pour the molten metal therein as by reversal of the position of the furnace as described in the Operhall Patent 2,806,271. The mold retains its strength under room or elevated temperatures so that the metal can be poured while the mold is cold but preferably after the mold has been preheated to elevated temperature, such for example as a temperature of 1600-2200 F. to facilitate fiow of the metal into innermost regions of the mold and to insure the production of suitable castmgs.

After the metal has been poured, the poured mold is removed from the furnace and set aside for solidification of the metal. Upon proper solidification, the mold can be broken up to remove the casting and then the latter can be processed in the usual manner for separation, cleaning, inspection and packaging to provide a finished product.

In accordance with one of the further concepts of this invention, it has been found that the conformity of the castings to the mold and the physical and mechanical properties of the casting can be materially improved if, instead of allowing the cast mold to cool in the open atmosphere, the cast shell mold is housed within an insu lated container whereby cooling is achieved at a slow and controlled rate for solidification and cooling of the poured metal. In the alternative, the cooling rate can be slowed under controlled conditions by placing the poured mold in the housing and filling the space about the mold with a flowable insulating material such as particles of exfoliated mica, bloated silica, Santocel and the Still further, the rate of cooling to the solidified state can be retarded by placing the poured mold in a heated space, such as a furnace, in which the temperature of the atmosphere can be controlled.

Having previously described the process from the time that the wax patterns are assembled in the cluster to the pouring of the molds, detailed description will now be made of concepts embodying additional features of this invention.

The dip coat compositions may deviate quite widely from the compositions set forth in the foregoing description since they may correspond to the formulations conventionally employed as a base for the application of stucco coats in conventional investment casting processes as defined in the aforementioned patents.

The improvements embodying the concepts of this invention find their support, however, in a combination which makes use of zircon flour as a filler in the dip coat compositions by way of displacement of flint or silica flour which is usually employed. It has been found that silica flour is subject to large inversions during temperature change between room temperature and 2000 F. whereby large contractions and expansions are caused to take place which introduce stresses Within the mold that cause deterioration or destruction. With silica, as with other materials, if the inversion stages are traversed quickly, cracks are formed in the mold part through which cast molten metal can escape. This condition would be highly objectionable in an investment casting, let alone a system wherein the mold part is adapted to be clamped to the furnace for pouring Without investment or other protective covering.

Both the alumina, used as the stucco, and the zircon used as the filler in the dip coating compositions embodying features of this invention have been found to be relatively free of these inversion characteristics by comparison with silica flour or silica. The expansion and contraction characteristics of zircon and of alumina are so low by comparison with other materials which have usually been employed, that the materials can be heated up rapidly to elevated temperaturefor maturing the mold or for preheating the mold without endangering the mold by reason of extreme expansions and contractions. It is for this reason that the Wax pattern with the described stucco coats thereon can be introduced into a furnace maintained at a temperature as high as 1800-2200 F. to achieve rapid heat-through of the shell for removal of the pattern and for proper maturing of the ceramic materials Within the space of 3-30 minutes. This is to be compared with the heating and baking cycle of 14 hours or more heretofore required for conventional in vestment molds. Thus the time factor in mold preparation can be greatly reduced and the output per unit space can be greatly increased thereby to conserve on the amount of fuel, equipment and space required for the preparation of molds in accordance with the practice of this invention.

By Way of further improvement in the process of mold preparation secured by the practice of this invention, more rapid heat-through enhances removal of the wax pattern and parts without endangering the mold. Slow heating of the molds, as necessitated by the conventional investment casting process, permits the wax pattern and parts to become heated through before elevated in temperature to a molten state whereby expansions are caused to take place while the Wax pattern and parts are still in a non-fiowable condition and fully confined within the investment. Expansion of the wax patterns while still in a solidified state results in the development of tremendous forces which often lead to loss in dimension of the mold and sometimes to the destruction and deterioration thereof to provide imperfections which cannot be determined prior to the removal of the castings from the mold. The more rapid heat-through permitted by the concepts embodying features of this invention provides for an entirely different operation. The outer portions of the wax pattern and parts are reduced to ambus statei prior to heating up the interior portions of the wax pat- V I erns and parts so that the wax in the outerportions can be eliminated from the moldprior to heating through the pattern. As a result, space'is madeavailable forsubse quent expansions which takelplace' upon heating up of the patterns and parts to minimize the development of destructive forcesin the'mold.

Nhilesilica can be used, as in conventional dip coating compositions, it has been found that it is possible to make use of much more rapid heating for maturing and preheating the mold when zircon is employed instead of This enables silica as the filler in these compositions.

better control over the casting process when used'for high temperature alloys. refractory which fuses within the temperature range of 31G0-3180 F. and which, is capable of-i'eaction with the slagging off of the silica provides for'contamination of the metalicasting :to produce an undesirable ancl unacceptable product. On the other hand, by usingzircof riium onzircon as the fillentemp'eratures upto 4000-, 4600 F. can be employed before fusion. Reaction conditions for slagging off are substantially avoided thereby to maintain dimensional stabilityiaud mold contour for the production of good castings,'even'with high tempera-- ture alloys. The combination of conditions of 'zircon filler and high temperatures for more'rapid maturing 'oi, the

mold and for casting high temperature alloys are' thusj employed to improve the casting process and the preparation of molds therefor.

{Instead otzircon, use can be made of finelyjdivided particles of flours of alumina, t itania, stabilized zircon, fused quartz, thoria, chromite, sillimanite, mullite, magnesia, vycor glass, or the like materials having similar inversion characteristics, good-"heat-shock properties, high fusion temperature andrelatively' low expansion characteristics at elevated temperatures Within the range from; room to 220051 It will be understood that the binder. component of the slip may comprise other conventional bindersemployed in the preparation of the stucco coats on wax patterns or patterns' formed of other low melting materials employed in investment castinga lWhen such'v other binder systems are employed, the zircon and the like;

filler can be incorporated in the amounts previously de-" scribed.

With reference to the dip coat slipsi'it is desirable to" make use of a 'fresh slip for the first dip coat since'the Silica also comprises a basic 7 appearns pimples s thefinal product. [on the other hand, if the stuccoistoo small, such for exampleas 'finer than 120 mesh, the stucco fails to develjopthe mechanical lock desired between the first and second dip coats and often leads to spalling'. In general, the balance between 'the mesh-size of the stuccoand'the thickness of the dip coat can best be determined'by trial and error but it will usually 'fall within the limitations described. The coverage of-the dip-coat subsequent to the first is enhanced if the stucco coat previously applied ispre-wet as by a wash coat formed of the dip coat composition markedly reduced in viscosity. Considerable-benefit is derived when a pre- Wet is employed in advance of the use of a dip coat of j-liigher viscosity than employed in previous dip coats.

For the second stucco coat, it is desirable to make use of a material'of about 50 mesh, Material larger than 7 35 mesh is too coarse and permits pockets to form the hot metal to develop a slaggy condition that enables silica to be washed from portions of the mold. This leaves an indefiniteness insome portions ofthe niold'andh20 exceed A inch'materialj Finer materials can be employed consistent with the desire to develop an adequate mechanical lock. .Ins-teadofIAlundum, use as a stucco coat can be made of zircon, sillimanite, 'mullite, vycor glass, tho'ria, chromite-, magnesia and the like materials having good heat-shock, good refractory characteristics,

flowflexpansion rate, andexp'ansionand. contraction characteristics close to those of the fillerin the dip coat.

An important concept of this invention resides in the means for providing an outer portion in the built-up shell which is capable of incipient fusion or a stage of vitrification at temperatures below the" vitrification temperature ofthe zircon to provide a skin barrier which has maturity at lower temperature to form a protective cover; heal any fractures or cracks which might develop in the mold duringburning out, preheating, and particularly during pouring of the metal, and markedly to strengthen the mold to impart mass integrity, which enables use of the shell .mol'd' without, support for casting molten metal therein,

with or without the accompaniment of pressure.

Brieflydescribed, the characteristics are achieved by the formul'ation'of an outer coating of the shell mold with feldspar present as" a component of the tiller in v the .dip coat. Feldspar vitrifies 'ata' temperature starting initial coating is more critical fromthe standpoint off thickness and character and bettercontr'olsfare available in a newly prepared batch. As a first dip coat, it is clesir-- able to provide for a thicknesson the patterns which will] enable the stucco to enter into the coating without penetration all the way through to the inner face of the mold,

if the coating is too thick, the extra/layer at the inner face'will not be mechanically heldstrong enough to pre vent separation from the restof the mold so that the inner layer of the dip coat can bepulled off'urider certain conditions to cause spalling. If the dip coat film is too thin,

the s ucco will penetrate through to cause the slip to flowinto the crevices between the stucco'partic'les and leave a rough inner face against which the metaliscast; Thus it is desirable to' provide for a balance ness and s'tucco For this purpose, it has been found best to provide for; a film thickness in t'he first stucco coat of about S IO mils with about 20 mils as a maximum and aboutjl mils as a minimum, and it is desirable, to makeluse of a between hi kr atabout l00O F. andcontinues onup to a temperature of about 2200 F. Thus the feldspar vitrifies alone and in combination with the siliceous materials present at the temperature conditions existing duringfiring and preheat- 7 ing to form a eutectic ceramic which is capableof sealing the pores? and cracks which are present or which otherwise tormiin the mold shellduring firing',- preheating or metal pouring.

As a resultof the sealing characteristics made available by-the feldspar in the outer. eutecticcoat or coats, the cracks are vblocked betoremetal' canflow therethrough during pouring with the-result that. the mold is saved,

metal is saved, and a higher yield of product is secured from the total of the metal poured. Scrap loss has been reduced from values as high as 4050%- to anaverage ofa few percent by the employment of the concepts forming the subject matter of this invention. i

;The eutecticoperates as a ceramic flux to secure. the particlesofstucco in the mold markedly to strengthen the mold shell tothe extent that the-mold'can be clamped stucco in a first stucco coat of about 60-80 mesh material.

If the mesh size of the stucco is consider-ably larger; capillaryactions developed will operate'to bleed moisture from i the dip coat composition and leave voids in the {face off the coating. These. Will'be filled-- by the cast metal-and on to the furnace for pouring the metal without investment or otherwise supporting the mold, and to the extent thatittis capable of withstanding normal forces existinginnormal handling and clamping as well as internal forces of the-liquid, heat and pressures which might be employed in' combination with. metal pouring. V

, Itisdesirable to make use of 'an amount of feldspar capable of imparting the desired strength characteristics described, but it is undesirable to make use of such concentration as will lead to excessive diffusion into the mold or will cause the development of hot tears. Feldspar in amount less than 0.05 part per one part by weight of Zircon or other filler in the dip coat has been found to be insufficient to impart the desired strength characteristics. When the amount of feldspar exceeds 0.25 part per one part by weight of zircon or other filler, the mold becomes so strong that it will lead to hot tears in operation and excessive diffusion into the mold may occur. It is pre ferred to make use of the feldspar in an amount within the range of 0.080.15 part by weight of the feldspar to one part by weight of the zircon or other filler in the dip coat or about 8-l5% by weight of the ceramic solids of the dip coat.

In operation, the feldspar has a tendency progressively to diffuse inwardly during firing. Since it is undesirable to have the feldspar pentrate into the inner surfaces of the mold, the time and temperature for heating should be balanced with the amount of feldspar to enable eute/ctic formation without complete penetration. Too much feldspar, that is, above the amount previously indicated, would enable progressive reaction to penetrate farther into the shell mold where undesirable conditions can be developed since the feldspar is capable of reaction with the metal while in the molten state. Further, it can form products which do not have the desired heat-shock resistance and thus the formation of such products should be limited to outer portions of the shell mold. Thus it is desirable to limit the addition of feldspar to dip coat compositions forming the outer coatings or the outer coating of the shell mold. While description has been made to the use of the outer coating as the eutectic coating containing the feldspar, it will be apparent that the eutectic coating can constitute one of the intermediate dip coats, which may or may not be stuccoed, on the condition that the eutectic coat is spaced at least two coats and preferably five or more coats from the face of the shell mold.

Practice of the concept described by modification of the dip coat to embody feldspar in an outer coating pro vides a shell mold having an inner face which is still highly refractory and incapable of reaction with the molten metal, and an outer coat which functions as a lower maturing coat which is capable of automatically scaling up pores and cracks that may form while at the same time wetting the particles with the eutectic type of binder markedly to increase the strength and toughness of the shell mold while, at the same time, reducing its heating time and temperature to maturity.

By varying the composition of the eutectic coating, temperature for maturing can be varied from 1000-2300 Though not equivalent, use can be made of iron oxide, borax or stannous chloride and the like low temperature vitrifiable inorganic materials instead of feldspar. The eutectic coat composition will generally be used at a lower viscosity than the conventional clip coats for stuccoing. In the composition described, the feldspar acts differently than borax in that it is capable of greater stability in suspension without upsetting the balance as compared to borax.

In the firing operation to mature the ceramic materials and to effect removal of the wax patterns from the shell mold, it is desirable to avoid the use of temperatures much in excess of 2200 F. because otherwise the materials will tend to form a glassy phase which adversely affects the mold. The minimum temperature is that temperature sufficient to activate the binder. For borate, temperatures as low as 500 F. could be used but, in general, it is preferred to make use of a temperature in excess of 1600 F. because it is difficult to effect the desired sequence in the elimination of the wax pattern and parts when firing at temperatures below 1600" F. Within the temperature range of l6002200 F., time is not an important factor. Very often 3 minutes is sufficient at 10 these temperatures, but it is preferred to heat for about 30 minutes. Heating for more than 30 minutes at the temperatures described is not harmful. Above 2200 F. the strength properties of the shell mold will be increased but reactions are possible which might cause excessive shrinkage in the shell mold.

In general, the thickness of the dip coats will increase from the inside out because of the increased porosity provided by the stucco. However, on the average, a shell formed of about five dip coats and an equal number of stucco coats will have a thickness of about inch and will be made up of about 60% by weight dip coat solids, 35% by weight coarse stucco of 1428 mesh, about 2% by weight of 20-mesh stucco from the first stucco coat, about 3% by weight 54-mesh stucco from the second stucco coat, feldspar will form about .5-1% by weight of the shell, and it will be located almost exclusively in the outer layer or layers before firing.

After the eutectic coat, it is desirable to dry the mold for about 815 hours before firing. After firing, the shell mold may be processed by grinding down the lip to provide a fiat surface for clamping to the top plate of the .elting furnace to achieve a sealing relationship therebetween which enables the shell mold to be clamped to the furnace whereby the furnace is subsequently inverted for pouring. After firing, it is preferred to also provide an additional eutectic coat on the outer surface of the shell to provide increased amounts of vitrification or incipient fusion and to provide additional feldspar for sealing the cracks and pores formed during the subsequent preheating operation and metal-pouring operation. As previously pointed out, the shell mold formed in accordance with the practice of this invention bears sufficient strength to enable it to be clamped directly onto the furnace for metal pouring as distinguished from the necessity to invest the shell within a ceramic support as in previous investment casting processes. The shell mold can be critically inspected prior to preheating and subsequent to preheating and prior to clamping the shell onto the melting furnace to discard defective shell molds and thereby avoid the waste of time and material in pouring unsuitable and unacceptable castings. Preheating can be carried out in the usual preheating furnace to raise the temperature of the mold to a desired high temperature for metal casting, thereby to insure complete flow of the metal into the innermost recesses of the mold with the result that acceptable castings of a uniform high quality can consistently be secured.

The cast metal shell can be removed from the furnace and set aside for cooling to solidify the metal but, as previously pointed out, castings having improved physical and mechanical properties and improved formation can be secured, when, in accordance with the practice of this invention, the poured shell mold is confined within a thermal insulating barrier during the cooling operation to solidify the metal or placed within a confined space subsequently filled with vermiculite or other particulate ther mal insulating material. Instead, the poured molds can be allowed to solidify in a furnace heated to elevated temperature to slow the solidification or the metal can be poured into a mold while housed within the furnace in which it is preheated or in another furnace.

It will be apparent from the foregoing detailed description that considerable advantages are capable of being derived by a metal casting process embodying features of this invention:

(1) It is possible to reduce the firing operation from the 16 or more hours characteristic of conventional investment casting processes to as little as from 3-30 minutes.

(2) The materials employed in the preparation of the shell mold for casting can be reduced from a representative value of $2.50 per cluster to as little as cents for an equivalent cluster. The foregoing applies to the mold l1 materials only since eliminated.

(3) The increased strength and shell'mold formed in accordance with the practice. of this 1 invention permits the shell mold to be clamped directly I to the furnace for pouring as distinguished from the necessity to invest the shell mold in conventional investment casting processes. 7

(4) The ability to clamp the shell'mold directly to the' furnace for pouring, coupled with the high strength characteristics of theshell mold at the elevated temperature the investment can be ,completely' mass integrity of j the V t materialcharacterized by low inversions during heating conditions existing, permits the use of, pressure in the a metal castingprocess to achieve more rapid and complete filling of the mold as distinguished from the inability to make use of pressure in other shell-molding processes.

(5) Because of the more refractory materials and their combination in a shell, mold pr'oducedin accordance with from ambient temperature to about ZOOO? R, drying the formed coatingslon the-pattern, introducing the pattern With/[he shelltforrned thereon intofa zone heated to a temperature above 1800"F. whereby heat rapidly penetrates throughthe shell to the pattern initially to reduce surfacepor-tions of thepattern to amolten state before the remainder of the patternis heated'to a temperature which would cause expansions of the pattern by an amount greater, than the portions reduced to a molten state and without. development of expansion forces within the shell the practice of this invention, it is possible to pour metal at higher temperatures Whichleads to the further possibility of pouring a greater variety of metals in the metal-casting process. use of a higher metal-pouring temperature is directly instrumental in achieving morecomplete flow of the metal in the mold to achieve moreuniform and better castings.

(6) Because of the good thermal'characteristics of the materials making up the shell mold, a wider range of temperatures is possible to enable pouring into shell molds both hot or cold. I a

"(7) The ability completely to inspect the shell mold in advanceof metal pouring is instrumental in reducing the scrap loss of the metal-castingrproces's by comparison products. g r v (8) By reason of the ability to'clam p the shell mold to the furnace, considerable conservation in heat can be effected by comparison with the transfer process heretofore required in pouring the metal from the melting furnace into a ladle and conveying the ladle to the mold for pouring. i

(9) It is-possible also to melt the metal in the furnace:

under a protective atmosphere and to maintain the desired of the metal" In addition, the ability to -makef as would cause breakage of the shell during rapid heat-up thereof, and continuing to expose the shell-pattern to the elevated temperature of the heated zone until the pat tern 'hasbeen eliminat'edfr'om the shell; 2. The method as-claimed in claim 1 in which the eat-disposable pattern is a wax pattern. 7

3., In themethod of producing a shell mold for met-a1 J casting, the steps of coatinga pattern of heat-disposable material with a slurry of a finely divided ceramic selected from the group consisting of zircon, alumina, titania, ,thoria', chromate, 'sillimanite, mullite, and magnesia, and characterized by. smallinversions by comparison With silica during heating fromambient temperature to, a temperatureof about 2000 F. whereby minimum expansions takepl'ace, stuccoing'the coated pattern with particulate ceramic materials selected from the group consisting of alumina and zircon, and characterized-by small inversions by comparison 'with silica during'heatingifrom ambient temperature to about 2000"; F. fwhereb'y minimum expansions occur, repeating the coating ofthe pattern alternately it with the slurry and'stucco to buildup a'shell of ceramic protective atmosphere during the'pouring' of thellmetal 7 into the shell mold when clamped, 'as previously described to the melting furnace. Such protective atmosphere can- 1 not be maintained where the metal must be removed from' the melting furnace for transfer by ladles to the mold for pouring. V i

' It will be apparent from the foregoing that We have provided an improved method and materials'for the production of metal castings.

formulations, their applications and in the operationswithwill be' understood that changes may be made in the details of materials, their 7 out departing from the spirit of the invention, especially as definedin the following claims.

"We claim: V a 1. In the method of producing a shell mold; for metal casting, the steps of coating apattern of heat disposable material with a slurry of a finelydivide'd ceramic selected from the'group consisting of zircon, alumina, titania,." thoria, chromite, sillimanite, mullite' and maguesia andi characterized by small inversio'ns by comparison with' silica during heating from ambient'temperature to a temf perature of about 2000" F. whereby mimimum expansions. ,7

.take place, stuccoing the coated pattern with particulate material characterized by low inversions during heating from ambient temperature to;about 2000 F., drying the formed coatings onrthe pattern, introducing the pattern With the shell formed thereon into a zone heated to a temperature within the range of 1800-2200 F. whereby heat rapidly penetrates through the shell to the pattern 5 initially "to reducesurface portions of the pattern to a moltenstate beforethe remainder, of the pattern is heated to a temperaturewhich would cause expansions of the pattern by an: amount greater than the portions reduced 7 to a molten state and without development of expansion forces within the shell as would cause breakage of the shell during rapid -heat-upthereof, and continuing to expose the shell-pattern to theelevated temperature of the heated zo'ne until the pattern has been eliminated from t the shelL. V j References Cited by the Examiner- UNITED STATES PATENTS 2,682,692 7 /54 Kohl 22l93 2,806,269 1 9/57 Dunlop 22196 2,932,864 4/60 Mellen et al 22-193 I i 7 OTHER REFERENCES .Glascast, published by CorningG1assWorks, Bulletin GC-'3; July 1957, 16'pages, pages 1'9:relied upon.

'LYONS, Primary Examiner; WILLIAMJ. STEPHENSON, MICHAEL v. BRINDISI, f 7' Examiners; 

1. IN THE METHOD OF PRODUCING A SHELL MOLD FOR METAL CASTING, THE STEPS OF COATING A PATTERN OF HEAT-DISPOSABLE MATERIAL WITH A SLURRY OF A FINELY DIVIDED CERAMIC SELECTED FROM THE GROUP CONSISTING OF ZIRCON, ALUMINA, TITANIA, THORIA, CHROMITE, SILIMANITE, MULLITE AND MAGNESIA, AND CHARACTERIZED BY SMALL INVERSIONS BY COMPARSION WITH SILICA DURING HEATING FROM AMBIENT TEMPERATURE TO A TEMPERATURE OF ABOUT 2000*F. WHEREBY MINIMUM EXPANSIONS TAKE PLACE, STOCCOING THE COATED PATTERN WITH PARTICULATE CERAMIC MATERIALS SELECTED FROM THE GROUP CONSISTING OF ALUMINA AND ZIRCON, AND CHARACTERIZED BY SMALL INVERSIONS BY COMPARSION WITH SILICA DURING HEATING FROM AMBIENT TEMPERATURE TO ABOUT 2000*F. WHEREBY MINIMUM EXPANSIONS OCCUR, REPEATED THE COATING OF THE PATTERN ALTERNATELY WITH THE SLURRY AND STUCCO TO BUILD UP A SHELL OF CERAMIC MATERIAL CHARACTERIZED BY LOW INVERSION DURING HEATING 